Hybrid cars combine two power trains that provide power in different ways for the propulsion of the vehicle. The properties of a combustion engine and an electrical motor are a particularly good fit, which is the reason why most of the hybrid cars today have such a combination. Today, parallel hybrid concepts in which the vehicle is propelled both by the combustion engine and the electrical motor are given preference over the serial hybrid concepts in which propulsion is provided basically by the electrical motor while the combustion engine generates the electrical current for loading the energy stores or directly for driving the electrical motor.
With the parallel hybrid concept there is the possibility to use at any given time, depending on the various operating states of the vehicle, the drive system that has the better efficiency at the given speed/power range. The electrical motor may be connected to the motor crankshaft in a variety of ways. For example, it may be connected via a clutch or directly to the crankshaft of the motor or may be coupled via a belt drive or gear.
The operation via the electrical motor may be limited for example to the range where little power is required and where a combustion engine is not very efficient, while higher power requirements are used to reload the electrical energy stores (which drive the electrical motor) via the combustion engine that is relatively efficient at that stage, plus via the dynamic operation of the electrical motor. In addition the combustion engine and the electrical motor may also impart motion in parallel, for example in order to increase the maximum torque of the entire power train.
Ideally the energy needed for propelling the vehicle at low speeds and low power requirements is gained from prior recuperation processes, i.e. energy recovery from the braking phases in which the required braking force of the vehicle is at least partially generated via the dynamic operation of the electrical motor in order to reload the energy stores. Because of road resistance, losses in the vehicle drive shaft and the loss-prone energy conversion chain mechanical→electrical/electrical→mechanical, when the charged energy stores are in balance only a portion of the total propulsion force can be provided over a given driving cycle by using the energy recovered in the braking processes. For these reasons alone it makes sense to use the recovered energy preferably only for the electrical motor propulsion during the operating phases, in which the combustion engine is not very efficient.
On the other hand, if the electrical motor and the combustion engine cannot be decoupled mechanically it is from an energy standpoint not advantageous to use only an electrical drive because the drag of the motor still has to be overcome, which would noticeably impair the overall efficiency of the power train. In such a case often a combined combustion engine/electrical drive is implemented, preferably with the electrical motor kicking in during operating phases with higher power requirements so that the combustion engine can continue to work in the relatively efficient ranges. However, since the energy recovered in the braking phases turns out to be in this case less because of the additional braking effect of the motor drag, less energy is available from these phases for propulsion by the electrical motor.
When designing a total system that is optimized for fuel consumption it is therefore above all necessary for the electrical motor to be able to yield, at least short-term and in dynamic operation, high electrical energy for the brake-energy recovery during braking phases. For this reason the maximum dynamic energy is a decisive criterion for the system design.
For completeness sake it should also be said that other requirements such as the cold-start capability of the combustion engine or the torque requirement for the boost operation in the range near full power must also be taken into consideration when designing the motor. However, when it comes to optimizing the overall system for the most efficient fuel consumption possible, these are initially relegated to the background. At the same time one must make sure that the electrical energy stores are able to absorb and give off the respective electrical power for the dynamic and mechanical operation.
Normally cycle-resistant batteries are used as electrical energy stores in hybrid cars. They have the advantage of relatively high volume and mass-related energy density, but they are limited in terms of the energy input and output that can be achieved. Furthermore, the life of the battery is limited by the energy throughput (i.e. absorbed and emitted energy), with the result that heavy cycling reduces the life of the battery.
Alternatively it may make sense to use other energy storage types for the energy storage system in a hybrid car. For example it is conceivable to use, instead of a battery, a capacitor store which can be stressed cyclically almost without limitation (i.e. with substantially higher cycle numbers than for batteries). So-called double-layer capacitors are preferred for use in hybrid concepts. Several of these capacitor stores must be switched in parallel in order to obtain sufficient capacity for the electrical power train at a specified voltage.
The disadvantage of a capacitor store versus a battery is above all the noticeably lower storable energy relative to volume (energy density). This means that a capacitor store may be able to provide only a relatively small energy amount in case of stress because of the limited space available in the vehicle.
The parallel use of a battery and a capacitor store combines the advantages of both energy storage systems with the result that the capacitor store handles most of the cyclical energy input and output processes and that battery use kicks in only during the rare and long-lasting electrical stress phases.
If the energy store is designed in the form of a capacitor store with switchable battery, whereby both stores can be switched in parallel at least for voltages that are below or equal to the rated voltage of the battery, most of the cyclical energy input and output processes can run via the capacitor store while the battery essentially guarantees that the combustion engine can be started when the capacitor store is empty and takes over the power network energy supply and the supply to the electrical motor in case of mechanical operation. Since the battery is only minimally cycled in this combination, a simple and inexpensive battery technology can be used, especially a battery technology based on plumbic acid technology. Alternatively a substantially smaller cycle-resistant battery on the basis of nickel metal hybrids or lithium ions may be used in parallel.
One must always also take into consideration the necessary electrical output when designing the electrical energy system of a hybrid car. At a given voltage the power must increase proportionately in case of a higher output, which in turn affects the design of the output electronics of the electrical motor and the cable profiles. One solution is to use higher voltages, reducing thereby the power and thus the output profile. Less power is also of advantage for the life of the battery. At higher voltages the electrical motor also shows a more favorable moment curve (high moment at higher speeds). However, this advantage is reduced at voltages above 60V by the measures required to protect from electric shock. This is why the voltage profile of a hybrid car represents an output-dependent compromise. Normally outputs up to about 4 kW are operated on a 12 V basis, outputs up to about 10 kW on a 48 to 60 V basis and outputs beyond 10 kW at voltages exceeding 60V.